1. Field of the Invention.
The present invention relates to signal conditioning circuitry and in particular to a circuit for conditioning a strain information signal.
2. Description of the Prior Art.
Strain measurements are normally given as a percentage, the percentage being a ratio of specimen elongation or compression to original length. In a typical tensile or compression test, there are two strain measurement points of particular interest. The first is known as the yield point.
The yield point marks the upper limit of the elastic strain range of a specimen. Throughout the elastic range, the modulus calculation (ratio of stress to strain) remains constant. At the yield point a discontinuity occurs and the ratio of stress to strain is no longer constant. The yield point for most metals occurs within a strain range of 0 to 1% although it may be as high as 5% for some nonmetal engineering materials. To make an accurate yield point determination, successive strain measurements must be taken and corresponding modulus calculations made. The difference between successive measurements is very small when the material is within its elastic range. High resolution of the strain information signal is therefore required. To achieve the necessary resolution high amplification of the strain information signal is needed. This results in the amplifier being useable for only a narrow range of strains before its output saturates.
The measurement of strain after yield and before failure is also of interest. Elongation of most materials fall within a range of 20-60% strain from yield to failure and thus much less resolution is required. The point of failure is called the ultimate elongation point and the elongation between yield and failure is called "ultimate elongation" herein. Specimen geometry normally does not allow a test to be performed with more than one extensometer. Ultimate elongation measurements are generally made by reassembling the broken specimen and measuring the length between prepunched gage marks. This method of measuring elongation is suspect. The prepunched gage marks often "smear" during the test and introduce error into the measurement. Many materials also exhibit notch sensitiveness and will consistently fail at the gage marks. The reassembly method is neither fast or accurate.
To summarize, yield point measurement and measurements of ultimate elongation are mutually exclusive under current test conditions. The high resolution needed to make accurate yield point measurements does not permit the amplifier range needed to measure final elongation. The wide range required to measure final elongation gives poor modulus and yield point results.